Secretory leukocyte protease inhibitor (SLPI) inhibits a range of serine proteases including human leukocyte elastase (HLE), cathepsin G, trypsin, and chymotrypsin. The major physiological role of SLPI is thought to be the protection of the mucosal epithelium against excessive proteolytic activity. Several studies have also shown that recombinant SLPI can inhibit HIV infection of macrophages and primary T-cells. Further studies have shown that rSLPI does not interact with the HIV aspartyl protease or reverse transcriptase but most likely binds to a host cell-surface molecule other than the primary HIV-1 receptor, CD4. Interestingly, several cell-surface serine proteases have been identified as possible complementary factors for HIV-1 infection. The purpose of our study is to determine whether there is a correlation between the anti-HIV and the serine protease inhibitory activities of rSLPI. The serine protease inhibitory siteof SLPI resides in the C-terminal domain. Previous studies have shown that oxidation of the methionyl resides of SLPI, all four of which reside in the C-terminal portion of the protein, decreases the affinity of SLPI for both chymotrypsin and trypsin. Additional studies in our laboratory indicate that the inhibitory effect towards HLE and cathepsin G is also affected. The relative decrease in inhibitory activity is a function of the oxidation conditions. The maximum decrease in inhibitory activity is observed for SLPI treated at a molar ratio of oxidant to protein of 20:1 and the amount of decrease in inhibitory effect is different for the four serine proteases. We would like to use ESI to determine the number of resides of oxidized methionine present in eachof our samples and if there is a mixture, an estimate of the ratios. We will monitor the anti-HIV activity of each of these samples by measurement of p24 production by macrophages infected with HIV-1Ba-L in the presence of rSLPI or the oxidized rSLPIs. These data wil lallow us to determine if the serine protease inhibitory activity of SLPI is also important for inhibition of HIV-1 infection of macrophages.